Opto-Mechanical Switching System

ABSTRACT

An optical switching system includes a two-dimensional arrangement of a plurality of switching elements. Each switching element includes an optical guiding structure which includes two pairs of waveguides; a driving arrangement for individually moving the switching elements between at least a first and a second position; and a first and a second input and a second output. When a generic switching element is moved in the first position, the first pair of waveguides connects a first input with a first output, and a second input with a second output; when a generic switching element is moved in the second position, the second pair of waveguides connects the first input with the second output, and the second input with the first output. In a first embodiment, the waveguides are provided on a disc shaped carrier and lie in the same plane, which disc is rotated. In a second embodiment, the two pairs of waveguides lie in different planes and the waveguide carrier plate is moved up and down for switching.

The present invention concerns an optical switching system, which can beused in particular in optical communications. More specifically, theinvention relates to an opto-mechanical switching system.

Modern communications networks are based on the employment of opticalfibers in order to handle the rapidly increasing demand for highertransmission rates. However, the enormous bandwidth of the opticalfibers can only be fully exploited once the networks become trulytransparent to bit rates and transmission protocols. In fact, at presenta conversion of optical signals into electrical signals (opto-electricalconversion) is necessary for executing a lot of operations required atnetwork nodes and electronic interfaces lead to network nodes nottransparent to the bandwidth and to the transmission bit rates.

In particular, in telecommunications the use of a component that permitsto dynamically connect different communications lines (a so calledcross-connect) is very important and, consequently, an opticalcross-connect is one of the key elements for transparent network nodes.However, at present in an optical communications network thecross-connection, for example between the different communicationslines, is not executed directly on the optical signals, but onelectrical signals obtained by the opto-electrical conversion.

Typically, a cross-connect is a switching system consisting of anarrangement of switching elements (switches) dynamically controlled forrouting the signals of the different communication channels towardsdifferent paths. Many types of switches have been proposed, such asswitches based on the specific optical properties of some materials; forexample, electro-optical switches exploit the Pockels effect, whileall-optical switches exploit second order optical nonlinearities, andholographic switches the photorefractive effect. Furthermore,opto-mechanical switches are based on the use of mini (micro) motors orMicro-Electro-Mechanics (shortly, MEM) for actuating a commutation bymeans of a specific rotational or translational movement of theswitches.

An optical switch is mainly characterized by a specific switching time,insertion losses, cross talk between the communication channels andpolarization dependent losses. For most switching purposes (as inswitched circuit communication networks) the relatively high switchingtime of the opto-mechanical switches (in the range from 10 ms to some100 ms) is considered adequate and, consequently, in many applicationsthe opto-mechanical switches are preferred because of the simplerimplementation, the lower fabrication costs and the easiercontrollability thanks to the use of standard technologies.

In general, in an opto-mechanical switching system the optical signalsare propagated either in free space, by multiple reflections onto micromirrors or by multiple deviations by prisms, or in optical guidingstructures, by multiple couplings into different optical waveguides orfibers.

An example of free-space cross-connect is described in A. C. M. Ruzzu etal., “Optoelectromechanical Switch Array With Passively AlignedFree-Space Optical Components”, Journal of Lightwave Technology, vol.12, No. 3, Mar. 2003, p. 664-671. This free-space cross-connect includesan array of movable micro mirrors and the optical signals (provided by aplurality of input optical fibers) undergo multiple reflections forbeing routed towards output optical fibers; the different controlledorientations of the micro mirrors permit the propagation along desiredoptical paths between the input optical fibers and the output opticalfibers.

The Applicant has observed that a free-space cross-connect has manyproblems, such as power losses and a beam waist increment in free-spacepropagation, and polarization dependent losses due to the angledependence of the reflectivity of the transverse electric and transversemagnetic modes. The power losses and the beam waist increment infree-space propagation limit the scalability of the cross-connect, i.e.the length of the optical paths inside the cross-connect and,consequently, the number of optical signals that can be routed.Furthermore, the beam waist increment can correspond to higher insertionlosses in the coupling of the optical signals into the output opticalfibers.

The French patent application No. 2479993 discloses an opto-mechanicalswitch comprising two pairs of optical fiber segments; the German patentapplication No. 30 12 450 proposes a further opto-mechanical switchhaving an optical guiding structure including two pairs of waveguides. Afirst pair of waveguides of the two proposed optical guiding structuresdeviates the direction of propagation of received optical signals,contrary to a second pair of waveguides. The optical signal from oneamong the waveguides of a first switch can be coupled into one among thewaveguides of a second switch by causing the switches to translate.

The U.S. Pat. Nos. 5,078,514 and 5,612,815 disclose opto-mechanicalswitches comprising MEM devices; in detail, the switches includes anintegrated optics guiding structure with a fixed part and a movable partconnected to the fixed part. The movable part includes waveguides, whichundergo a translational movement thanks to a deformation induced by anelectrostatic transducer.

The actuation of the translational movement in those switches impliesstresses of the movable part and a very critical controllability, asalso recognized in the U.S. Pat. No. 5,612,815.

Considering a two-dimensional arrangement of the switches as disclosedin the French patent application No. 2479993, in the German patentapplication No. 30 12 450 and in the U.S. Pat. Nos. 5,078,514 and5,612,815, a plurality of input optical fibers can be accommodated infront of a respective switch and, at the same time, a plurality ofoutput optical fibers can collect the optical signals from respectiveswitches. The Applicant observes that the proposed optical guidingstructures do not permit the implementation of a two-dimensionalarrangement of these switches that grants a flexible configuration andan easy controllability of the possible optical paths. In fact, thecontrol of the configurations of the signal optical paths in thetwo-dimensional arrangements of these switches is not simple, since forobtaining a new configuration it may be required to translate most of,or even all, the switches. Furthermore, in some cases the exploitationof a free-space propagation or a guided propagation in further opticalguiding structures between two switches is necessary, accordinglyleading to bulky two-dimensional arrangements.

In the U.S. Pat. No. 4,653,849 an optical switching assembly isdisclosed, comprising a distribution of a number P of plates and anumber N of translating (rotating) switches incorporating N waveguides.Each plate receives the respective optical signal from one respectiveinput optical fiber or waveguide and the received optical signal iscoupled into all the waveguides of the plate. The switches are arrangedin such a way that each waveguide of one switch receives the respectiveoptical signal from a respective plate; the translational movement ofeach switch permits the optical coupling between the desired waveguideand one respective output optical fiber or waveguide for transmittingthe desired optical signal thereinto. Thus, the switching assemblyimplements a P×N cross-connect, in which the optical signals travelthrough controlled guided optical paths.

The Applicant has observed that the switching assembly disclosed in thatdocument is hardly configurable with the increase of the number N ofoutput optical fibers, because of the complexity of a single plate orswitch that comprise a great number of waveguides.

In view of the state of the art outlined in the foregoing, it has beenan object of the present invention to overcome the above-mentioneddrawbacks. In particular, it has been an object of the present inventionto provide an optical switching system that ensures easycontrollability, high scalability and, at the same time, a simpleimplementation.

In order to achieve this object, according to an aspect of the presentinvention, an optical switching system as set out in the claim 1 isproposed.

Summarizing, an optical switching system includes a two-dimensionalarrangement of a plurality of switching elements adapted to routing aplurality of optical signals provided at inputs of the optical switchingsystem towards respective outputs of the optical switching system.

Each switching element includes:

an optical guiding structure comprising two pairs of waveguides, eachwaveguide having an input and an output end faces;

a driving arrangement for individually moving the switching elementsbetween at least a first and a second positions; and

a first and a second inputs couplable to said input end faces, and afirst and a second outputs couplable to said output end faces, each ofsaid first and second inputs being optically coupled to either an inputof the switching system or an output of an adjacent switching element,and each of said first and second outputs being optically coupled toeither an output of the switching system or an input of a furtheradjacent switching element.

When a generic switching element is moved in the first position, thefirst input is coupled to the input end face of a first waveguide of thetwo pairs of waveguides, and the first output is coupled to the outputend face of the first waveguide, thereby an optical signal received atthe first input is routed to the first output; the second input iscoupled to the input end face of a second waveguide of the two pairs ofwaveguides, and the second output is coupled to the output end face ofthe second waveguide, thereby an optical signal received at the secondinput is routed to the second output.

When a generic switching element is moved in the second position, thefirst input is coupled to the input end face of a third waveguide of thetwo pairs of waveguides, and the second output is coupled to the outputend face of the third waveguide, thereby an optical signal received atthe first input is routed to the second output; the second input iscoupled to the input end face of a fourth waveguide of the two pairs ofwaveguides, and the first output is coupled to the output end face ofthe fourth waveguide, thereby an optical signal received at the secondinput is routed to the first output.

The first, second, third and fourth waveguides are integrated opticalwaveguides, and the two-dimensional arrangement is a matrix arrangementwith a number M of rows of switching elements and a number N ofcorresponding columns of switching elements, the first input of ageneric switching element being optically coupled to either an input ofthe switching system or the first output of a switching element of asame row, and the second input of a generic switching element beingoptically coupled to either a further input of the switching system orthe second output of a switching element of a same column.

In an embodiment of the present invention, the plurality of opticalsignals is provided by first optical elements, each first opticalelement being coupled to one respective input of the optical switchingsystem, and the plurality of optical signals routed towards the outputsof the optical switching system is received by second optical elements,each second optical element being coupled to one respective output ofthe optical switching system.

Each switching element may further include one optical device coupled toat least one among the first and second end faces of the first, second,third and fourth waveguides for focusing/collimating an optical signal.

In an embodiment of the present invention, the switching system includesa further optical device coupled to at least one among the first andsecond optical elements for focusing/collimating an optical signal.

A control unit may be further provided for, for dynamically controllingthe driving arrangement of the plurality of switching elements.

According to an embodiment of the present invention, the two pairs ofwaveguides of the switching elements lie in a same plane and the drivingarrangement causes the optical guiding structure to rotate about an axisof the plane for moving the respective switching element between thefirst or the second positions.

The input and output end faces of the first, second, third and fourthwaveguides of each switching element may be angularly separated ofapproximately 45°.

In another embodiment of the invention, the first and second waveguidesof the switching elements lie in a first plane and the third and fourthwaveguides lie in a second plane, the first and second planes beingparallel, and the driving arrangement causes the optical guidingstructure to translate perpendicularly to the two planes for moving therespective switching element between the first and the second positions.

The arrangement of the plurality of switching elements may beaccommodated in a box-shaped housing, which may include grooves forreceiving the first and the second optical elements.

According to another aspect of the present invention, an opticalcommunications network node as set forth in claim 11 is provided,receiving a plurality of optical signals, each one provided to arespective input of an optical switching system, the optical switchingsystem routing each received optical signal towards a respective outputof the switching system. The optical switching system is according tothe first aspect of the invention.

Further features and the advantages of the present invention will bemade clear by the following description of some embodiments thereof,provided purely by way of non-limitative example, description that willbe conducted making reference to the attached drawings, wherein:

FIG. 1 is a top view of an optical switching system according to anembodiment of the present invention (without a cover);

FIG. 2 is a side view of the optical switching system in cross sectionalong line II-II of FIG. 1;

FIG. 3 shows schematically a longitudinal section of an integratedoptics guiding structure of an opto-mechanical switching elementexploited in the optical switching system of FIG. 1;

FIG. 4 illustrates a transverse section, along the line IV-IV, of theintegrated optics guiding structure of FIG. 3;

FIG. 5A is a top view of the opto-mechanical switching element in a barconfiguration;

FIG. 5B is a top view of the opto-mechanical switching element in across configuration; and

FIGS. 6A, 6B and 6C show an elevation sectional view and two top-planeviews (at different heights) of an opto-mechanical switching elementaccording to an alternative embodiment of the present invention.

Referring to the drawings, FIG. 1 is a top view of an optical switchingsystem 100 according to an embodiment of the present invention. Albeitnot limitatively, the switching system 100 can be used for dynamicallyrouting optical signals through different optical paths, in particularin optical communication networks. For example, the optical switchingsystem 100 can be used in optical communications networks intended tosupport Wavelength Division Multiplexing (shortly, WDM) opticalcommunications, in which case the switching system 100 can be used fordynamically routing optical signals transported through respective WDMchannels.

The switching system 100 includes a two-dimensional arrangement, or anM×N matrix, of opto-mechanical switching elements (hereinafter simplyreferred to as switches) 105 _(i), with i=1, . . . ,M×N; for the sake ofsimplicity, in the present description it is supposed that M=N=2 so thatthe switching system 100 comprises four switches 105 ₁-105 ₄. Eachswitch 105 _(i) comprises a disk-shaped element (hereinafter referred toas disk) 110, e.g. of silicon, with a diameter, for example, roughlyfrom 6 to 10 mm, carrying an integrated optics guiding structure.

The generic switch 105 _(i) further includes a driving mechanicalarrangement (not visible in FIG. 1, but shown schematically in FIG. 2)for causing a rotation of the switch 105 _(i) about the axis thereof (asdescribed in detail in the following).

The integrated optics guiding structure of the switches 105 _(i)includes four waveguides g1, g2, g3 and g4, particularly coplanarwaveguides, for implementing 2×2 switches (i.e., switches with twoinputs and two outputs). Each waveguide g1-g4 has a first end face and asecond end face disposed on the outer peripheral edge of the disk 110.In particular, the waveguides g1 and g2 have the first and the secondend faces at the opposite extremes of two mutually orthogonal diametersof the disk 110. Each waveguide g3, g4 has the first and the second endfaces at the extreme of two respective radii of the disk 110 forming anangle of approximately 90°. In addition, the radius corresponding to thefirst end face of the waveguide g1 and the radius corresponding to thefirst or the second end faces of the waveguide g4 form an angle ofapproximately 45°; the radius corresponding to the second end face ofthe waveguide g1 and the radius corresponding to the first or the secondend faces of the waveguide g3 form an angle of approximately 45°.Consequently, the waveguide g1 is intersected by the waveguides g2, g3and g4. Furthermore, the central part of the waveguides g1 and g2 isbent, for going round a central hole in the disk, having for example adiameter of about 1 mm, while the central part of the waveguides g3 andg4 is bent for smoothly joining the two ending parts thereof.

The bending radius of the bent part of the waveguides g1, g2, g3 and g4depends on the refractive index contrast of the integrated opticsguiding structure; for example, with a relatively high refractive indexcontrast of roughly 4.5% the bending radius can be of the order of amillimeter (for example, of about 1-1.2 mm). Accordingly, the size ofthe disk 110 depends on the value of the bending radius and, then, onthe value of the refractive index contrast. In addition, the physicaldimensions of the driving arrangement (e.g., minimotors) may affect thesize of the disk.

Spherical lenses 130 (for example, of glass or even of a plasticmaterial), hereinafter referred to as ball lenses 130, are placed intorespective seats provided in the disk 110, in front of a respectivefirst or second end faces of the waveguides g1, g2, g3 and g4.Consequently, the ball lenses 130 of the switches 105 _(i) are arrangedin such a way that the radii of the disks 110 corresponding to twoconsecutive ball lenses 130 form an angle of approximately 45°. The balllenses 130 can have a diameter of about few hundreds of micrometers (forexample, 150-300 μm) and a refractive index roughly in the range from1.46 to 2. The ball lenses 130 focus the optical beams into, orcollimate the optical beams received from, the waveguides g1, g2, g3 andg4. The ball lenses 130 are held in place within the respective seats,being fixed to the disk 110 by an appropriate adhesive compound (such asa resin).

A frame 135 (for example, of steel or of silicon) is adapted tosupporting the M×N matrix of switches 105 _(i); the frame 135 has atleast M grooves 140 on a first side S1 thereof and at least N grooves140 on a second side S2, consecutive to the first side S1. Preferably,the frame 135 has also at least M grooves 140 on a third side S3,opposite to the first side S1, and at least N grooves 140 on a fourthside S4, opposite to the second side S2. The grooves 140 are adapted toaccommodating an optical fiber 145i, 150i and are arranged in such a waythat optical fibers 145 _(i), 150 _(i) can be aligned with thewaveguides g1-g4 of respective switches 105 _(i).

Input single mode optical fibers 145 ₁, and 145 ₂ are accommodated andfixed inside the respective grooves 140 of the first side S1;preferably, further input single mode optical fibers 145 ₃ and 145 ₄ areaccommodated and fixed inside the respective grooves 140 of the fourthside S4 of the frame 135. An input fiber 145 ₁ is placed on the firstside S1 of the frame 135 and an input fiber 145 ₃ is placed on thefourth side S4 both in front of a switch 105 ₁; an input fiber 145 ₂ isplaced on the first side S1 in front of a switch 105 ₂ and an inputfiber 145 ₄ is placed on the fourth side S4 of the frame 135 in front ofa switch 105 ₄.

Output single mode optical fibers 150 ₁ and 150 ₂ are accommodated andfixed within the respective grooves 140 of the second side S2 of theframe 135; output single mode optical fibers 150 ₃ and 150 ₄ areaccommodated and fixed within the respective grooves 140 of the thirdside S3 of the frame 135, opposite to the first side S1. The outputfibers 150 ₁ and 150 ₂ are placed in front of the switch 105 ₂ and theswitch 105 ₃, respectively; the output fibers 150 ₃ and 150 ₄ are placedin front of the switch 105 ₄ and the switch 105 ₃, respectively.

Along an internal edge 170 of the frame 135, seats adapted to containingfurther ball lenses 130 are provided at the end of each groove 140;these further ball lenses 130 are exploited for focusing the opticalbeams into, or for collimating the optical beams received from, theoptical fibers 145 ₁-150 ₄ positioned in the grooves 140. These balllenses 130 are aligned with the longitudinal axis of the groove 140(i.e., the longitudinal axis of the optical fiber 145 ₁-150 ₄) and canface the ball lenses 130 of the switches 105 _(i).

Furthermore, the arrangement of the switches 105 _(i) is such that theball lenses 130 of a switch 105 _(i) can face the ball lenses ofadjacent switches 105 _(i) and of the frame 135. The distances betweentwo aligned ball lenses 130 are preferably of the order of few tens ofmicrometers, so as to limit the free-space path of the optical beams andconsequent losses.

The frame 135 is provided with a threaded hole at each corner and at thecenter thereof, and screws 155 are used for fixing the frame to asupport (not visible in FIG. 1).

The Applicant observes that a switch comprising an integrated opticsguiding structure is preferable than an opto-mechanical switchcomprising optical fiber segments for saving of occupied area, forexample, inside a network node. In addition, the integrated opticspermits to exploit automated fabrication processes, thus bringing to cutproduction costs.

Similar considerations apply if the element carrying the integratedoptics guiding structure of the switches has a different shape (i.e.,not necessarily the shape of a disk). The waveguides may have analternative pattern, e.g. their end faces may be angularly separated ofdifferent angles. Furthermore, the arrangement of the switches may bedifferent, for example, the switches may be arranged in lines along afirst direction and along a second direction, the first and seconddirection being not orthogonal. The grooves in the frame for the opticalfibers can have an alternative arrangement, according to thearchitecture of the arrangement of the switches. The optical fibersfixed into the grooves can be of different type and can be in adifferent number, i.e., only a fraction of the grooves can accommodateoptical fibers. Furthermore, alternative optical waveguide may be usedin place of optical fibers, or the optical signals may be coupled intothe integrated waveguides of the switches after a free-space path.

Alternatively, the lenses for focusing or collimating the optical beamsare of different type, or they may be dispensed for, e.g., when thewaveguides are properly tapered or other beam collimating solutions arerealized at their end portions, or when the switches are immersed in amatching fluid. For example, also Fresnel or GRaded INdex (GRIN) lensescan be used; preferred solutions include the use of hemi-sphericallenses fixed to the end faces of the waveguides with their flat surface.The exploited lenses can have different sizes and a different refractiveindex depending on the switching system architecture and on theintegrated optics guiding structure (or on an arrangement of opticalfiber segments).

It has to be pointed out that the switching system according to thepresent invention can be exploited in general in applications in whichoptical signals need to be routed through different optical paths. Inaddition, the switching system according to the present invention istransparent to the received optical signal, and, particularly, to thebit rate and to the transmission protocols.

Considering now FIG. 2, a side view of the switching system 100 isschematically illustrated in cross section along line II-II of FIG. 1(the elements corresponding to those depicted in FIG. 1 are denoted withthe same reference numerals and their description is omitted for thesake of simplicity). The frame 135 leans onto a support 205, and a cover210 is placed and fixed (in a way to be described later on) onto theframe 135.

The driving mechanical arrangement of each switch 105 _(i) comprises asmall electric motor 215, here referred to as minimotor 215, fixed tothe support 205; the minimotor 215 actuates the rotational movement of arelatively small shaft 218, such as to insert into the central hole ofthe disk 110, e.g. of a diameter of about 1 mm. A coupling joint 220joins the end of the shaft 218 and a rotary table 225 for transmittingthe rotational movement thereto. A ball bearing 230 between the rotarytable 225 and the frame 135 supports and guides the rotary table 225 soas to ensure that the rotational movement is planar. The disk 110 isplaced onto the rotary table 225 in such a way that the axis of theshaft 218 coincides with the axis of the disk 110 and the disk 110 mayrevolve according to the rotation of the shaft 218 about the axisthereof.

A pushing pin 227 is inserted into the central hole of the disk 110 andinto a central hole of the rotary table 225; a bias spring 240 pushesthe pin 227 downwards, so that the pushing pin 227 can press the disk110 onto the rotary table 225, ensuring that the disk 110 moves togetherwith the mechanical arrangement during a rotation. The vertical pushingaction of the bias spring 240 is adjusted by a screw 245 inserted intothe cover 210 and stopped by a nut 250; the head of the screw 245 andthe nut 250 are external to the switching system 100.

A guide pin 228 projects from the rotary table 225 for limiting therotation of the switches 105 _(i) to desired angles. The guide pin 228slides within an arc-shaped groove 260 formed in the frame 135,corresponding to an arc of a circumference concentric to the disk 110.

The guide pin 228 cooperates with the two end walls of the respectivearc-shaped groove 260 for angularly limiting the rotational movement ofthe switch 105 _(i). The length of the arc-shaped groove 260 is suchthat, when the movement of the guide pin 228 is stopped by either one orthe other of the end walls of the groove 260, four ball lenses 130 ofthe switch 105 _(i) are positioned so as to face corresponding balllenses 130 of the adjacent switches 105 _(i) and of the frame 135. Eachguide pin 228 allows the switch 105 _(i), and consequently the disk 110,rotating of at most an angle of approximately 45° about the axisthereof. Alternatively, the arc-shaped groove 260 is longer and theguide pin 228 allows the disk 110 rotating of at most an angle ofapproximately 135°.

The minimotors 215 are controlled by a control unit 255 (shown onlyschematically in FIG. 2), which controls the relative position of theswitches 105 _(i) and actuates the rotation, when required, for properlyconfiguring the switching system 100. Consequently, by means of thecontrol unit 255 it is possible to directly control the optical path ofan optical signal inside the switching system 100. By exploitingcommercially available minimotors 215 (e.g. an EC 6 maxon EC motormodel) it is possible to reach a switching time of the order ofmilliseconds (for example, five milliseconds) depending also on the sizeof the disk 110.

Alternative embodiments may provide that the mechanical drivearrangement has a different structure and the disk is fixed onto therotary table by means different than the pushing pin and the biasspring; furthermore, the rotational movement of the switch may belimited by a device different than a guide pin sliding within a groove.Similar considerations apply if Micro-Electro-Mechanical elements (MEM)are integrated with the disk in place of the mechanical arrangementsdriven by the minimotors.

With reference to FIG. 3, a longitudinal section of the integratedoptics guiding structure integrated on the disk 110 is schematicallyshown. The integrated optics guiding structure of the disk 110 isobtained, for example, by a known fabrication process, consisting ofdifferent stages, which will be briefly discussed in the following.

The integrated optics guiding structure is formed, preferably, on asilicon (Si) substrate 310 of the disk 110, having a thickness of about600 μm. Directly laying on the substrate 310 the integrated opticsguiding structure presents, for example, a silica (SiO₂) lower claddinglayer 312 of thickness in the range from about 7 to about 15 μm, overwhich a silicon oxinitride (SiON) core 315 of thickness of about 1.8-2.5μm leans. A silica upper cladding layer 320 of thickness in the rangefrom about 7 to about 15 μm covers the whole integrated optics guidingstructure.

In front of an end face of the core 315 a seat for a ball lens 130 isetched in the silicon substrate 310 in such a way that the longitudinalaxis of the core 315 is substantially aligned to an axis of the balllens 130; consequently, the ball lens 130 can focus (collimate) anoptical beam 330 into (received from) the core 315.

However, the integrated optics guiding structure can be fabricated bydifferent planar technologies exploiting alternative material, bothcrystalline and amorphous, assuring a suitable refractive index contrastand, where necessary, the possibility of etching the seats for thelenses. Alternatively, when a high refractive index difference is notnecessarily required, commercially mature planar technologies relying ona low index contrast typically of about 0.75% can also be exploited. Forexample, the waveguides can be Ge-doped silica waveguides having anindex contrast ranging from relatively low values, such as 0.7%, tohigher values, such as 3.5%; alternative waveguides can be obtained alsoby deposition of Ge Boron-Silicate Glass (GeBSG), adapted to implementwaveguides with a relatively high index contrast, such as 4.5%.

Considering now FIG. 4, a transverse section, along line IV-IV of FIG.3, of the integrated optics guiding structure integrated on the disk 110is schematically illustrated (the elements corresponding to thosedepicted in FIG. 3 are denoted with the same reference numerals andtheir description is omitted for the sake of simplicity). In particular,the SiON core 315 presents a rectangular section of width of about 2-2.5μm.

The principal stages of an exemplary fabrication process for theformation of the integrated optics guiding structure are describedhereinbelow with reference both to FIG. 3 and FIG. 4.

The fabrication process is executed on a silicon wafer typically havinga thickness of 600 μm. Preferably, for obtaining the lower cladding 312of the guiding structures, about 7-15 micrometers of the thickness ofthe silicon substrate 310 undergo a thermal oxidation or a SiO₂ layer ofthe same thickness is deposited by other standard processes; in thisway, the wafer presents a layer of silica directly laying on the siliconsubstrate 310. Successively, an upper layer, for example, of SiON ofabout 1.8-2.5 μm is formed, e.g. by a Plasma-Enhanced Chemical VaporDeposition (PECVD) process (other techniques being possible,particularly other chemical processes or, for example, also thermalprocesses).

SiON permits the implementation of a guiding structure with a relativelyhigh refractive index difference between the core 315 and the lowercladding 312 of about 4.5% (typically, a high index contrast is higherthan 2.5% and preferably lower than 10%). The bending losses in aguiding structure increase for shorter bending radius, but decrease forhigher refractive index difference. Thanks to the use of SiON, anoptical guiding structure with an index contrast of about 4.5% can havea bending radius of, for example, 1-1.2 mm and resulting bending lossesof about 0.01 dB/mm. Preferably, the size and the refractive indexdifference of the waveguides are chosen in such a way as to ensure themonomodality of the waveguides.

The definition of the waveguide patterns in the SiON layer is made bymeans of a photolithographic process; a layer of photo-resist isdeposited over the whole wafer, a mask is aligned with the wafer, andthen the wafer is exposed to a suitable radiation for defining thewaveguide patterns. The exposed photo-resist is developed andselectively removed, and SiON (or, in general, the material exploitedfor implementing the core of the waveguides) in excess with respect tothe waveguide pattern is etched away. The etching of the SiON bringspreferably to a rectangular transverse section of the core 315, as shownin FIG. 4.

Successively, a silica layer 320 is deposited onto the wafer, e.g. usinga Chemical Vapor Deposition (shortly, CVD) process, for example a PECVDprocess.

Finally, a center narrow hole and the seats for the ball lenses 130 (notvisible in the drawings) is obtained, preferably, by a furtherphotolithographic process exploiting a dry etching.

It is observed that alternative fabrication processes, as well asalternative materials can be used, for example, for producingsemiconductor waveguides. For example, GeBSG can be deposited by a LowPressure CVD process in place of SiON and BSG in place of SiO₂(obtaining an index contrast up to about 5%). Alternatively, also thesilica lower cladding can be obtained by a CVD process instead ofthermal oxidation of the silicon substrate. Those skilled in the artwill recognize that the size of the waveguides varies accordingly to therefractive index contrast.

FIG. 5A is a top view of the switch 105 _(i) in a so-called barconfiguration. An input optical signal is provided to the switch 105_(i) by one of the waveguides g1-g4 of an adjacent switch 105 _(i), orby an input fiber 145 ₁-145 ₄ (not shown in the drawing); in the barconfiguration, the input optical signal is coupled into the waveguidesg1 and/or g2.

In detail, an optical beam 530i carrying a first input optical signal iscollimated by a first input ball lens 130 i′ placed in front of the endof a waveguide g1, g4 of an adjacent switch 105 _(i) or of an inputfiber 145 ₁, 145 ₂. After a short free-space path, since in the barconfiguration the first input ball lens 130 i′ is aligned with thelongitudinal axis of the waveguide g1, the beam 530i is focused by asecond input ball lens 130 i″, placed in front of the first end face ofthe waveguide g1 of the switch 105 _(i) and the input optical beam 530 iis coupled into the waveguide g1 (fiber-to-waveguide coupling orwaveguide-to-waveguide coupling). Similarly, an input optical beam 535 icarrying a second input optical signal can be simultaneously coupledinto the waveguide g2 by means of the alignment between the waveguide g2and a further input ball lens 130 i′, and by another input ball lens 130i″ placed in front of the end of waveguide g2.

The coupled optical signal travels through an in-waveguide path g1, g2inside the disk 110 towards the second end face of the waveguide g1, g2.Output optical beams 540 o and 545 o, carrying first and second outputoptical signals corresponding to the first and second input opticalsignals, are collimated by first output ball lenses 130 o′ placed infront of the second end faces of the waveguides g1 and g2, respectively.After a further, short free-space path, the output beam 540 o is focusedby a second output ball lens 130 o″, coinciding with a first input balllens 130 i′ in front of the waveguide g1 or g3 of the adjacent switch105 _(i) or, in case, the output optical fiber 150 ₃ or 150 ₄.Similarly, the output beam 545 o is focused by the second output balllens 130 o″, coinciding with the first input ball lens 130 i′ in frontof the waveguide g2 or g4 of the adjacent switch 105 _(i) or in front ofthe output optical fiber 150 ₁ or 150 ₂. The output optical beams 540 oand 545 o are coupled into the output optical fibers 150 ₁-150 ₄(waveguide-to-fiber coupling) or into the waveguides g1-g4 of adjacentswitches 105 _(i), if the respective first output ball lenses 130 o′ arealigned with the longitudinal axes thereof.

It is observed that in the bar configuration the direction of the outputbeams 540 o and 545 o does not deviate with respect of the direction ofthe input beams 530 i and 535 i.

FIG. 5B is a top view of the switch 105 _(i) in a so-called crossconfiguration. In the cross configuration the input optical signal iscoupled into the waveguides g3 and/or g4; the cross configuration isobtained by a counterclockwise rotation of the switch 105 _(i) of anangle of 45° (or, alternatively, by a clockwise rotation of 135° asdescribed above with reference to FIG. 2) with respect to theabove-described bar configuration.

The input optical beam 530 i, collimated by the ball lens 130 i′, isfocused by the second input ball lens 130 i″, located in front of thewaveguide g3 of the switch 105 _(i), and, then, the first opticalsignal, carried by the input optical beam 530 i, is coupled into thewaveguide g3. The coupled optical signal travels through the guided pathg3 inside the disk 110 towards the first output ball lens 130 o′, placedin front of the end of the waveguide g3. By means of the aligned firstand second output ball lenses 130 o′ and 130 o″ the first output opticalsignal, corresponding to the first input optical signal and carried bythe output beam 545 o emerging from the lens 130 o′, is coupled into theoutput optical fiber 150 ₁, 150 ₂ or the waveguide g2, g4 of theadjacent switch 105 _(i). Similarly, the second input optical signalcarried by the optical beam 535 _(i) can be simultaneously coupled intothe waveguide g4 and a corresponding second output signal carried by theoutput beam 540 o is collimated by the output ball lenses 130 o′ placedin front of the second end face of the waveguide g4. The output beam 540o can be focused by the second output ball lens 130 o″, coinciding withthe first input ball lens 130 i′ in front of the waveguide g1, g3 of anadjacent switch 105 _(i) or, in case, of the output optical fiber 150 ₃,150 ₄.

Differently from the bar configuration, in the cross configuration thedirection of the output beam 540 o, 545 o forms an angle of 90° withrespect to the direction of the corresponding input beam 535 i, 530 i.For bringing the switch 105 _(i) back to the bar configuration from thecross configuration, the switch 105 _(i) may be rotated clockwise of anangle of 45°.

The switch 105 _(i) can route two optical signals at the same time, thetwo optical signals being received by both the waveguide g1 and thewaveguide g2 or by both the waveguide g3 and the waveguide g4. Dependingon the position reached by the generic switch 105 _(i), an opticalsignal is received by the waveguide g1 or g2 and does not deviate itsdirection, or, alternatively, is received by the waveguide g3 or g4 anddeviates its direction of an angle of 90°.

Referring back to FIG. 1, optical signals are preferably fed to theswitching system 100 by the input fibers 145 ₁ and 145 ₂, while theoutput fibers 150 _(i) and 150 ₂ collect the optical signals dynamicallyrouted by the switching system 100. The input fibers 145 ₃ and 145 ₄ andthe output fibers 150 ₃ and 150 ₄ can be, for example, exploited formonitoring the performances of the switching system 100. For example, ifthe switching system 100 is used in the context of a WDM network, eachinput fiber 145 ₁, 145 ₂ provides an optical signal of a respective WDMchannel, the optical signals being preliminary separated by an opticalmultiplexer; similarly, each output fiber 150 ₁, 150 ₂ collects theoptical signal of the desired WDM channel.

The arrangement of the switches 105 _(i) in the switching system 100 issuch that it is possible, by means of the controlled rotation of thedisks 110, to align a plurality of ball lenses 130 of the switches 105_(i) and of the frame 135, so as to obtain a prevalently guided opticalpath from an input fiber 145 ₁, 145 ₂ to a chosen output fiber 150 ₁,150 ₂. Each optical path is obtained by successivewaveguide-to-waveguide couplings of the optical signal.

Let it be supposed that the optical signal propagated by the input fiber145 ₁ has to be routed towards the output fiber 150 ₂, and, accordingly,the optical signal propagated by the input fiber 145 ₂ towards theoutput fiber 150 ₁. Considering the switch 105 ₁ in bar configuration,the ball lens 130 in front of the input fiber 145 ₁ is aligned with thelongitudinal axis of the waveguide g1 of the switch 105 ₁ for couplingthe received optical signal into the waveguide g1. The longitudinal axisof the waveguide g1 of the switch 105 ₁ has to be aligned with thelongitudinal axis of the waveguide g3 of the switch 105 ₄, for couplingthe optical signal into the waveguide g3 (the switch 105 ₄ is in crossconfiguration). Similarly, the optical signal is then coupled into thewaveguide g2 of the switch 105 ₃, for being collected into the outputfiber 150 ₂ (the switch 105 ₃ is in bar configuration). In this case,the switch 105 ₂ has to be in cross configuration for coupling theoptical signal received from the input fiber 145 ₂ into the waveguide g3and, then, into the output fiber 150 ₁.

It is observed that for obtaining the optical path between the inputfiber 145 ₁ and the output fiber 150 ₂ it is sufficient to bring incross configuration the switch 105 _(i) in the row of the matrixcorresponding to the input optical fiber 145 ₁ and in the columncorresponding to the output optical fiber 150 ₂ (in this case the switch105 ₄). A similar consideration applies to the optical path between theinput fiber 145 ₂ and the output fiber 150 ₁.

Alternatively, an optical path between the input fiber 145 ₁ and theoutput fiber 150 ₂ is obtained by the counterclockwise rotation of 45°of the switch 105 ₁ and of the switch 105 ₃ for reaching the crossconfiguration. In this case the optical signal from the input fiber 145₁ is coupled into the waveguide g3 of the switch 105 ₁, successively,into the waveguide g4 of the switch 105 ₂ and, then, into the waveguideg3 of the switch 105 ₃. The configuration of the switch 105 ₄ is notrelevant.

In addition to the two above-considered optical paths from the inputfibers 145 ₁ and 145 ₂ to the output fibers 150 ₂ and 150 ₁,respectively, two further alternative optical paths can besimultaneously configured in the switching system 100 from the inputfibers 145 ₁ and 145 ₂ to the output fiber 150 _(i) and 150 ₂,respectively. In detail, the switches 105 ₁ and 105 ₃ have to be incross configuration, while the switch 105 ₂ has to be in barconfiguration. In this way, the optical signal from the input fiber 145₁ is coupled into the waveguide g3 of the switch 105 ₁, into thewaveguide g2 of the switch 105 ₂ and, then, into the output fiber 150 ₁.Similarly, the optical signal from the input fiber 145 ₂ is coupled intothe waveguide g1 of the switch 105 ₂, into the waveguide g3 of theswitch 105 ₃ and, then, into the output fiber 150 ₂. Also in this case,the configuration of the switch 105 ₄ is not relevant.

It is observed that a number M (in the example M=2) of switches 105 _(i)in cross configuration is sufficient in a M×M matrix to obtain all thedesired optical paths between the input optical fibers and the outputoptical fibers, particularly, it is sufficient having one switch incross configuration in each row and column of the M×M matrix. In thisway, for varying the optical paths of the signals inside the opticalswitching system it is required to actuate the movement of at most M+Mswitches. In detail, considering the worst case in which the opticalpaths of all the received signals have to be varied, for each row andcolumn the switch previously in the cross configuration is brought inthe bar configuration, while the switch corresponding to the new outputof the signal, previously in the bar configuration, is brought in thecross configuration. As a consequence, the movement of at most twoswitches has to be actuated for each row and column irrespective of thematrix dimensions and, accordingly, the controllability of the opticalswitching system is very easy.

Similarly, for example when a monitoring has to be performed, inputoptical signals can be received also at the input fibers 145 ₃ and 145 ₄and routed also towards the output optical fibers 150 ₃ and 150 ₄.Alternatively, all the input and output optical fibers 145 _(i) and 150_(i) can be exploited during operation of the switching system 100.

Furthermore, it has to be considered that the switching system 100 isbi-directional, i.e., the optical fibers 145 ₁-145 ₄ referred to asinput optical fibers might be exploited as outputs of the switchingsystem 100, and the optical fibers 150 ₁-150 ₄ referred to as outputoptical fibers might be exploited as inputs.

It is observed that the optical signal, before being coupled into awaveguide g1-g4 or into an output fibers 150 ₁-150 ₄, covers the shortfree-space paths between two ball lenses 130, i.e., free-space paths ofonly about tens of micrometers. In a free-space path of such a smalllength the power losses and the increase of the beam diameter of theoptical signal are very low. Typically, the insertion losses undergoneby an optical signal in a fiber-to-waveguide or a waveguide-to-waveguidecoupling are of about fractions of dB.

Considering FIGS. 6A, 6B and 6C, an elevation sectional view and twotop-plane views (at different height) of an opto-mechanical switchingelement 605 according to a preferred alternative embodiment of thepresent invention are shown.

According to this preferred embodiment, each switch 605 comprises ablock 610, preferably, of silicon for carrying an integrated opticsguiding structure, and an electro-mechanical driving arrangement adaptedto actuate a vertical translation of the switch 605 (as described in thefollowing).

Similarly to the previously described embodiment, the integrated opticsguiding structure of the switches 605 includes four waveguides g1, g2,g3 and g4, which are in this case arranged on two different longitudinalplanes of the block 610. The waveguides g1 and g2 lie in an upper planewith respect to the plane of the waveguides g3 and g4; the waveguides g1and g2 are substantially straight and orthogonal to each other, thewaveguides g3 and g4 are bent and do not intersect. Each waveguide g1-g4has a first end face and a second end face disposed on the edge of theblock 610. In particular, the first end faces of the waveguides g1 andg3 are on a first side face of the block 610, while the first end facesof the waveguides g2 and g4 are on a second side face consecutive to thefirst side. The second end faces of the waveguide g1 and g4 are on athird side face of the block 610 opposite to the first side, while thesecond end faces of the waveguides g2 and g3 is on a fourth side face.The waveguides g1, g2, g3 and g4 have the first and the second end faceon the median of the respective side of the block 610.

The integrated optics guiding structure, integrated in the block 610, isformed on a substrate 611, preferably, of silicon. A lower cladding 612,for example, of silica is directly laying on the substrate 611 and acore 615 of the waveguides g3 and g4 is formed on the lower cladding612. A first thick upper cladding 620 covers the whole of the lowercladding 612 and the core 615, and a core 622 of the waveguides g1 andg2 is formed on the first upper cladding 612; a second upper cladding630 covers the whole integrated optics guiding structure. Two magnets635 are associated, for example, with the top and the bottom faces ofthe block 610, respectively. Alternatively, only one magnet 635 isplaced in the substrate 611 in a suitable position intermediate betweenthe top and the bottom faces of the block 610, e.g. in correspondence ofthe center of the block 610 (as illustrated in dash-and-dot lines inFIG. 6A).

The fabrication process is executed on a wafer, preferably, of silicon,similarly to the process described with reference to the embodiment ofFIGS. 1-5B. In this case, the core 615 of the waveguides g3 and g4 andthe core 622 of the waveguides g1 and g2 are formed in different stagesof the fabrication process. In particular, the core 622 of thewaveguides g1 and g2 is formed on the first upper cladding 620 of thewaveguides g3 and g4, if necessary, after a planarization of the firstupper cladding 620 (for example, by a Chemical-Mechanical Planarizationprocess), and the second upper cladding 630 is realized by deposing anadditional silica layer. Those skilled in the art will recognize thatthe upper cladding 620 has to be of a thickness sufficient to decouplethe optical signals traveling through the waveguides g1, g2, g3 and g4of the two different planes. Finally, a layer of ferromagnetic materialis deposited over the block 610 for obtaining the magnet 635 centeredwith respect to the top face of the block 610.

The mechanical arrangement of a switch 605 cooperates with a frame 645,similar to that described with reference to FIGS. 1-2. The block 610 isinserted in a respective, e.g. cylindrical, sliding seat 690 formed inthe frame 645, in which the block 610 is axially slidable guided by thewalls of the seat 690.

As shown in FIGS. 6B-6C, four axial grooves 655 extend longitudinally atthe periphery of the block 610. The whole structure is immersed in amatching fluid, such as glycerin, that fills the not occupied space ofthe seat 690; the matching fluid has a refractive index close to that ofthe guiding structure materials, for limiting the losses incurred duringwaveguide-to-fiber and waveguide-to-waveguide couplings of the opticalsignals. Moreover, the matching fluid acts also as lubricant for themovement of the block 610 inside the hollow volume and the grooves 655permits to the matching fluid to flow freely. It has to be observed thatthe use of a matching fluid and, if necessary, of waveguides havingtapered ending portions allows avoiding the exploitation of optics, suchas lenses, for coupling optical signals into the waveguides and opticalfibers.

Two electromagnets 650, each one in front of a respective magnet 635,are aligned to the axis of the block 610. The translational movement ofthe switch 605 along the axis of the block 610 is actuated by switchingone electromagnet 635 at the time depending of the desired direction,thus attracting the magnets 635 associated with the block 610. Thetranslational movement of the switch 605 is such that the optical signalcan be coupled from the waveguides g1, g2, g3 and g4 of a switch 605into the selected waveguides g1, g2, g3 and g4 of adjacent switches 605.A rod 665 is properly inserted into the block 610 for cooperating toguiding the movement of the block 610 into the direction parallel to theaxis thereof.

However, the concepts of the present invention apply also when the blockof the switch has a different shape, and particularly a different crosssection, when the grooves are different in number and when themechanical arrangement includes more than one rod. Alternatively, thetranslational movement is actuated in a way different from anelectromagnetic actuation, for example by an electromechanical actuationother than electromagnetic, or in a way altogether different from anelectromechanical actuation.

It can be appreciated that the present invention provides a switchingsystem of simple implementation thanks to the use of standardtechnologies.

The easy controllability of the switching system according to thepresent invention is due to the simple architecture of the integratedoptics guiding structure of the switches permitting both a bar and across configuration. The architecture of the integrated optics guidingstructure of each switch permits a deviation of the optical signals ofan angle of 90° and a switching system comprising a two-dimensionalarrangement of such switches results very flexible.

Furthermore, this simple architecture of the integrated optics guidingstructure of the switches is not a limit for the scalability of theswitching system, since millimetric sizes of the switches are possibleand, in addition, two different optical signals at the time can becoupled into the waveguides of each switch. The switching system permitsa high scalability also thanks to that prevalently guided paths areencountered by the optical signals.

Naturally, in order to satisfy specific requirements, a person skilledin the art may apply to the solution described above many modificationsand alterations all of which, however, are included within the scope ofprotection of the invention as defined by the following claims.

1-11. (canceled)
 12. An optical switching system, comprising atwo-dimensional arrangement of a plurality of switching elements adaptedto routing a plurality of optical signals provided at inputs of theoptical switching system toward respective outputs of the opticalswitching system, wherein each switching element comprises: an opticalguiding structure comprising two pairs of waveguides, each waveguidehaving an input and an output end faces; a driving arrangement forindividually moving the switching elements between at least a first anda second position; and a first and a second input couplable to saidinput end faces, and a first and a second output couplable to saidoutput end faces, each of said first and second inputs being opticallycoupled to either an input of the switching system or an output of anadjacent switching element, and each of said first and second outputsbeing optically coupled to either an output of the switching system oran input of a further adjacent switching element, wherein: when ageneric switching element is moved in the first position: the firstinput is coupled to the input end face of a first waveguide of the twopairs of waveguides, and the first output is coupled to the output endface of the first waveguide, whereby an optical signal received at thefirst input is routed to the first output; and the second input iscoupled to the input end face of a second waveguide of the two pairs ofwaveguides, and the second output is coupled to the output end face ofthe second waveguide, whereby an optical signal received at the secondinput is routed to the second output, and when a generic switchingelement is moved in the second position: the first input is coupled tothe input end face of a third waveguide of the two pairs of waveguides,and the second output is coupled to the output end face of the thirdwaveguide, whereby an optical signal received at the first input isrouted to the second output; and the second input is coupled to theinput end face of a fourth waveguide of the two pairs of waveguides, andthe first output is coupled to the output end face of the fourthwaveguide, whereby an optical signal received at the second input isrouted to the first output, the first, second, third and fourthwaveguides being integrated optical waveguides, and the two-dimensionalarrangement being a matrix arrangement with a number of rows ofswitching elements and a number of corresponding columns of switchingelements, the first input of a generic switching element being opticallycoupled to either an input of the switching system or the first outputof a switching element of a same row, and the second input of a genericswitching element being optically coupled to either a further input ofthe switching system or the second output of a switching element of asame column.
 13. The optical switching system according to claim 12,wherein the plurality of optical signals is provided by first opticalelements, each first optical element being coupled to one respectiveinput of the optical switching system, and the plurality of opticalsignals routed toward the outputs of the optical switching system isreceived by second optical elements, each second optical element beingcoupled to one respective output of the optical switching system. 14.The optical switching system according to claim 12, wherein eachswitching element further comprises one optical device coupled to atleast one among the first and second end faces of the first, second,third and fourth waveguides for focusing/collimating an optical signal.15. The optical switching system according to claim 14, wherein theswitching system comprises a further optical device coupled to at leastone among the first and second optical elements for focusing/collimatingan optical signal.
 16. The optical switching system according to claim12, further comprising a control unit for dynamically controlling thedriving arrangement of the plurality of switching elements.
 17. Theoptical switching system according to claim 12, wherein the two pairs ofwaveguides of the switching elements lie in a same plane and the drivingarrangement causes the optical guiding structure to rotate about an axisof the plane for moving the respective switching element between thefirst or the second positions.
 18. The optical switching systemaccording to claim 17, wherein the input and output end faces of thefirst, second, third and fourth waveguides of each switching element areangularly separated by approximately 45°.
 19. The optical switchingsystem according to claim 12, wherein the first and second waveguides ofthe switching elements lie in a first plane and the third and fourthwaveguides lie in a second plane, the first and second planes beingparallel, and the driving arrangement causes the optical guidingstructure to translate perpendicularly to the two planes for moving therespective switching element between the first and the second position.20. The optical switching system according to claim 12, wherein thearrangement of the plurality of switching elements is accommodated in abox-shaped housing.
 21. The optical switching system according to claim20, wherein the box-shaped housing comprises grooves for receiving thefirst and the second optical elements.
 22. An optical communicationsnetwork node capable of receiving a plurality of optical signals, eachnode provided with a respective input of an optical switching system,comprising the optical system according to claim 12, the opticalswitching system routing each received optical signal toward arespective output of the optical switching system.